Elasticsearch tracks the indexing activity of each shard. Shards that have not
received any indexing operations for 5 minutes are automatically marked as inactive. This presents
an opportunity for Elasticsearch to reduce shard resources and also perform
a special kind of flush, called synced flush. A synced flush performs a normal flush, then adds
a generated unique marker (sync_id) to all shards.

Since the sync id marker was added when there were no ongoing indexing operations, it can
be used as a quick way to check if the two shards' lucene indices are identical. This quick sync id
comparison (if present) is used during recovery or restarts to skip the first and
most costly phase of the process. In that case, no segment files need to be copied and
the transaction log replay phase of the recovery can start immediately. Note that since the sync id
marker was applied together with a flush, it is very likely that the transaction log will be empty,
speeding up recoveries even more.

This is particularly useful for use cases having lots of indices which are
never or very rarely updated, such as time based data. This use case typically generates lots of indices whose
recovery without the synced flush marker would take a long time.

To check whether a shard has a marker or not, look for the commit section of shard stats returned by
the indices stats API:

The Synced Flush API allows an administrator to initiate a synced flush manually. This can be particularly useful for
a planned (rolling) cluster restart where you can stop indexing and don’t want to wait the default 5 minutes for
idle indices to be sync-flushed automatically.

While handy, there are a couple of caveats for this API:

Synced flush is a best effort operation. Any ongoing indexing operations will cause
the synced flush to fail on that shard. This means that some shards may be synced flushed while others aren’t. See below for more.

The sync_id marker is removed as soon as the shard is flushed again. That is because a flush replaces the low level
lucene commit point where the marker is stored. Uncommitted operations in the transaction log do not remove the marker.
In practice, one should consider any indexing operation on an index as removing the marker as a flush can be triggered by Elasticsearch
at any time.

It is harmless to request a synced flush while there is ongoing indexing. Shards that are idle will succeed and shards
that are not will fail. Any shards that succeeded will have faster recovery times.

POST /twitter/_flush/synced

The response contains details about how many shards were successfully sync-flushed and information about any failure.

Here is what it looks like when all shards of a two shards and one replica index successfully
sync-flushed: